During my earliest days, I remember I first commented Kisa-sensei, I said I like sumire. He saidit was unusual that someone said they Like Sumire.
Well, if I was in Sou's place then Sumire (as my Nee-chan) would be out by default...
But in general?
I'd have no problem being together with Sumire, I actually like her quite a bit. Sure, she's quirky, totally horny and kinda acts like human trash, but that's just half the picture and exactly what's so refreshing about her. Unlike other girls who don't want to show their bad sides, Sumire has her failures and she shows them - it's "what you see is what you get". And when I look at her then she's absolutely gorgeous - on par with Aya.
So what if she cannot cook (I can, no worries) and I don't mind her lounging around like a NEET when she's at home - Sumire is a looker, I would happily enjoy the view. And she may act like a girl failure, yet she is anything but. She is a college student, so this girl has no issues working. Sumire really cares for the one she likes and while she might be a bit possessive she's clearly not an eerie yandere or totally overprotective. She knows where to draw the line and reflects on her mistakes.
Sumire would be a fine girlfriend and in that world she'd be a strong partner.